This invention relates generally to damped mechanical structures and methods of their manufacture and, more particularly, to damped structures employing viscoelastic materials. There are a number of applications of mechanical structures in which there is a requirement to damp out oscillations caused by varying mechanical loads, either axial loads, bending and shear loads, or torsional loads. A particularly important category of applications includes structures for use in aerospace vehicles. In these applications, there are three major requirements: stiffness, lightness in weight, and the ability to absorb energy by damping any oscillations that would otherwise be present. Fiber composite materials can easily satisfy the stiffness and weight requirements, but the design of an appropriate self-damping structure is more difficult to achieve.
Viscoelastic materials offer an efficient means of dissipating energy. Although these materials have poor stiffness, they provide an energy dissipative effect in direct proportion to the elastic force applied to them. Since a viscoelastic material is soft, it is typically made in thin layers and held between constraining layers of stiffer elastic materials. Relative motion of the elastic layers induces a shear load in the viscoelastic material and thereby dissipates energy. This is known as constrained layer damping, and is generally limited to damping flexural motions of the base member. The constraining layer acts through the viscoelastic layer to produce a dynamic stiffness in parallel to the elastic stiffness of the base member.
Another prior approach to viscoelastic damping is series damping. In a series damper, the entire load in a member is made to pass as a shear load through the viscoelastic material. This allows the dissipation of a large amount of energy, but no parallel elastic load path is provided through the viscoelastic material. Also, failures can often occur in series damping viscoelastic materials, and surface sealing of the materials can be difficult. Sealing is important because many viscoelastic materials are inherently unstable and, if not sealed from exposure to the environment, will "outgas" vapors from their surface. This process not only degrades the desirable elastomeric properties, but also contaminates other materials in the near vicinity. Accordingly, a sealing layer is usually provided over viscoelastic materials, to minimize exposure to sunlight and oxygen.
A variation of the constrained layer approach is to employ a segmented constraining layer, allowing both extensional and flexural motions to be damped. This is discussed in "Constrained Layer Damping Effectiveness of Members under Vibratory Extensional Loadings," by Stanley S. Sattinger, presented at the ASME Design Engineering Conference in Cincinnati, Ohio, 1985, paper no. 85-DET-134. The segmented constraint can dissipate only a limited amount of energy, as the underlying elastic structure will always carry a significant portion of the load.
A good solution to the difficulties posed by the prior art was proposed in U.S. Pat. No. 5,030,490, entitled "Viscoelastic Damping Structures and Related Manufacturing Method," issued in the names of Allen J. Bronowicki and Abner Kalan. The invention disclosed and claimed in the aforementioned patent resides in a composite structure of viscoelastic and elastic materials, in which dynamic loads are directed along a path that passes repeatedly through a viscoelastic layer, to maximize damping in the structure. Although the tubular form of the structure provides significantly improved viscoelastic damping properties for various types of dynamic loads, it has the disadvantage of having an internal viscoelastic layer, which adds to manufacturing difficulties. The present invention is an improvement over the structure disclosed in the aforementioned patent.